1. Field of the Invention
The present invention relates to a microwave tunable device, and more particularly, to a tunable capacitor using a ferroelectric thin film and a phase shifter using a photonic band gap (PBG) structure.
2. Description of the Related Art
PBG structures were first introduced in the field of optics, but have recently been widely used in high frequency devices such as radio frequency (RF) and microwave devices.
Phase shifters are widely used in microwave systems to steer electron beams and shift the frequency of a radio signal. A phase shifter is an essential component of a phase array antenna system for producing a beam pattern and steering a beam. Phase shifters using a ferroelectric thin film have low manufacturing costs because they are simple to manufacture compared to ferrite/semiconductor phase shifters and provide high switching speed due to high-speed polarization. In particular, because ferroelectric phase shifters have low microwave loss due to the low loss factor of a ferroelectric thin film, much research on ferroelectric phase shifters is being actively conducted to replace conventional ferrite/semiconductor phase shifters that suffer from high microwave loss at higher frequency.
Typical ferroelectric phase shifters are mainly classified into coplanar waveguide (CPW) phase shifters, loaded line phase shifters, and reflective phase shifters including a tunable capacitor mounted at the end of a directional coupler. However, typical phase shifters require many experiments to extract design parameters. Another drawback of phase shifters is that they suffer from large insertion loss variation because characteristic impedance and phase shift vary according to an applied voltage. Thus, there is a need for a phase shifter having a novel structure to overcome the drawbacks.